Blender Git Loki

Bugfix #4649

Three issues:
- When saving a file, without extension added, and no filename provided,
the saving code received the directory name only. That's a potential
danger of getting directories deleted.
Added in the saveover() function a check for this, and return an error
when you try to save over a directory.

- Screendump did not add file extensions yet, when indicated todo so.

- Screendump code was duplicating all image type cases, whilst we have a
nice BKE_write_ibuf() call for that now. (Bug was that this code did not
check for BMP, saving the file in default format.)